1. Field of the Invention
This invention generally relates to a process for obtaining textured coatings from Photo-curable urea-containing compositions. More particularly the invention relates to photopolymerization of ethylenically unsaturated compounds using a combination of an aromatic ketone photosensitizer and a multifunctional urea compound.
2. Description of the Prior Art
It is known that photopolymerization processes have achieved considerable industrial importance particularly in situations where thin films or layers need to be cured in short times. Typical of such situations are the curing of clear coatings or overprint varnishes, colored coatings, inks, adhesives, printing plates, and the like. Such products are used in a variety of commercial end uses. Although the prime interest is for such thin-film technology, the importance of thick-film or thick-section radiation-cured technology is growing and increasing in importance. Thick-section technology is important in the area of sealants, adhesives, shaped parts, printing plates, fiber glass- or carbon/graphite fiber-reinforced products, and the like.
The use of aromatic ketones such as benzophenone, benzophenone derivatives, and the like as photosensitizers in the photopolymerization of ethylenically unsaturated compounds is also known in the art. However, some photosensitizers, such as benzophenone, will not function when used alone and require the presence of a second compound, a synergist. In addition, markedly faster polymerization rates can be obtained by the use of combinations of such aromatic ketones with synergists than is the case when such photosensitizers are used alone. The synergists usually are low molecular weight tertiary amines, ureas, or amides.
Combinations of photosensitizers and synergists that are commonly used include mixtures of one or more aromatic ketones, one of which is benzophenone, isopropylthioxanthone, or the like and a low molecular weight tertiary amine such as dimethylethanol amine, triethylamine, bis-(4,4'-dimethylamino)benzophenone (Michler's ketone), or the like. The combination of photosensitizer and a synergist is termed a photoinitiator system or a photoinitiator combination. Such combinations are said to function through an intermolecular hydrogen abstraction mechanism in which ultraviolet light is absorbed by the aromatic ketone which interacts with an amine or other suitable low molecular weight nitrogen-containing compound that has an alpha-hydrogen and rapidly forms an excited complex. This complex, or "exiplex" as it is often termed, undergoes rearrangement with the transfer of a hydrogen atom from the amine causing the amine to become the free radical initiator molecule that causes rapid polymerization of acrylates and other appropriate ethylenically unsaturated compounds. If used alone, the amine or other nitrogen-containing compound will not effect polymerization. Although this mechanism is felt to be true, this invention is not limited by it or any other described mechanism.
The textured finish is a visual or tactile surface or appearance of something that is characterized by a woven or interwoven appearance which in turn is characterized by the number of weaves or strands per unit of surface. The nature of the textured finish may be very coarse, very fine, or variations between these limits. Also, the textured finish may have excellent optical clarity, translucency, or opacity. In a general sense, it means a rough-appearing surface rather than a smooth appearing surface. Textured coatings are both useful and decorative. They can provide a nonglare surface that can be used in various ways including coatings for business machines, control lights, light emitting diodes, and other end uses. In addition, textured coatings tend to hide surface dirt or soil as well as slight surface imperfections.
It is also known that various finishes for substrate coatings are desirable. These finishes include matte or flat, glossy, wrinkle or hammer-tone, or intermediate variations of such finishes. In the photo-curing industry, texturing is difficult to achieve and technology has been developed on special processes for preparing textured, ultraviolet light-cured coatings. In a general sense, this technology relates to control of the surface, the curing environment, and the light source type, intensity and number of exposures. The overall results are workable but complicated processes.
According to U.S. Pat. No. 4,421,784 textured finishes have been achieved in a variety of ways including by using particular formulations or additives such as flatting agents, allowing a time interval between exposures to ultraviolet light, curing all of the coating except the surface and then subjecting the uncured surface to ultraviolet light in air until the surface is fully cured, using ultraviolet light to obtain first a partial cure of the exterior and the interior in air and completing the cure, and curing in air all of the coating except the surface and then irradiating the surface in an inert atmosphere, and curing first the interior of the coating.
U.S. Pat. No. 4,421,784 is an improvement over the known processes and involves applying an ultraviolet light curable formulation with a viscosity of at least 50 centipoise to a substrate and increasing the coating viscosity by exposing the coated substrate to ultraviolet light with a wavelength of 180 to 275 nm in an inert atmosphere for a period of time sufficient to initiate texturing on the coating surface. After exposure, the coated substrate is maintained in a space devoid of ultraviolet light for a period of time sufficient for the surface to texture. The surface-textured coating is then exposed to ultraviolet light with a wavelength of about 180 to 400 nm in an inert or an air atmosphere until the coating is cured.
U.S. Pat. No. 4,483, 884 is an improvement over U.S. Pat. No. 4,421,784 cited above in that it provides for dark colored and light colored, nonreflective and reflective to ultraviolet light backgrounds to be used during the curing process.
U.S. Pat. No. 4,485,123 is yet another improvement over U.S. Pat. No. 4,421,784 cited above in that it provides for dark colored and light colored infrared adsorbent and reflective backgrounds to be used during the curing process.
The currently known methods for texturing radiation curable coatings have many deficiencies. They require the use of specially designed coating equipment with a prechamber for initiation of cure that contains an ultraviolet light source of a special low wavelength, a dark area or tunnel in which essentially no ultraviolet light is present, and a third area in which the coating is subjected to full spectrum ultraviolet light to complete the cure. In addition, it is necessary for an inert environment to be used in the prechamber. Improvements in this process require the use of special dark or light colored ultraviolet light or infrared reflective or nonreflective background surfaces or substrates to be used during the texturing process.